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I have inspected your form: http://www.jotformeu.com/form/51745043955358 and I see that you have payment field inside of it. When I selected product inside of your form Total and sub-total values are shown.
It is not possible to write in total and sub-total field. Values from other fields are passed to total and sub-total.
Can you please explain in more details what you are trying achieve?
Also this articles can help you to create a payment form.
Hope this will help. Let us know if you need further assistance.